In central air conditioning systems where conditioned air is supplied at a single temperature, it is often necessary and/or desirable to change over from heating to cooling and vice versa. Such changeovers can be long-term, as in the case of seasonal changes or short-term, as in the case of warming up an overcooled building prior to the arrival of the work force. For example, after a weekend or holiday during which the minimum cooling condition overcooled the building in the absence of the cooling load supplied by the personnel, lights and equipment, it is desirable to warm the building long enough to reduce the overcooling to an acceptable level. The thermostatic response is, however, correct without change for only one mode of operation. If, for example, the thermostatic response is correct for cooling, the supplying of heated air will encounter a minimum or no flow condition in an undercooled zone or area and dissatisfaction causing overheating if the zone temperature is above the cooling set point. Obviously, neither condition is satisfactory or energy efficient.